Publicly published video is not the same as public domain video. Check the video and uploader pages carefully for words like "Creative Commmons" or "Public Domain". If you don't find an explicit statement of license, the work probably remains under full copyright protection. I suggest you write an email to the person who uploaded the video, asking for their provenance (documentation of origin) or simply asking for permission to use the material.

When in doubt, ask permission. It will save you from some serious headaches in the future.Here's a link to where I get my stuff for projects:http://www.footagefirm.com/free-nature-and-animals-sound-effects.html

It's not "free" (just shipping and handling), but for the price you get quite a great deal of resource material to use.